Mapping/texturing inside of a box
CYCLES RENDER

To give a texture only to the inside of the box you may use a Backfacing output of the Geometry node.


Unwrap your mesh and set the nodes as pictured below. Use a Backfacing output as a factor for two materials/textures.




BLENDER RENDER

In Blender Internal you may also use the material nodes to achieve this effect.


Unwrap your mesh and give it a new material. Check the Use Shader Nodes box.


Now add two other materials- one for the inside and one for the outside of the box.


In the Node Editor window set up the nodes as pictured below.


You may also add just one material and create two others within it using nodes. Unwrap your mesh and add a new material. Check the Use Shader Nodes box.


Set up the material nodes as pictured below.